In modern aircraft, a number of aircraft engines are provided, as well as an auxiliary power unit (APU). Conventionally, the main engines are started by air turbine starters, requiring valves, ducts, and other equipment in the tightly packed engine nacelle. Further, the APU is typically started by a DC machine which only provides a useful function for a very short period of time over each APU engine cycle. Also, the starter motor is a high maintenance item owing to the use of brushes therein.
Of late, more electric aircraft have been studied whereby central hydraulic and pneumatic systems are removed and electric power is utilized to start engines, provide air conditioning as well as cabin pressurization and fresh air flow, retract landing gear provide local hydraulic centers, and provide some forms of de-icing. Additionally, variable frequency power generation capability has been proposed for loads, as opposed to the traditional matching of power conversion to the loads, i.e., fixed frequency 400 Hz. and/or motor drives. Coupled with this is the need for fault tolerance and minimum, yet sufficient, redundancy.
When the power ratings of generators coupled to the engines become sufficient, main engine electric start becomes an option. This capability allows significant amounts of equipment to be removed from the aircraft, particularly in tightly packed areas, such as the engine nacelle.